Implantable microphones for use with cochlear implants and other hearing systems typically require an implantable converter for receiving the sound reaching the ear of the patient and converting the sound into electrical signals for further processing in the hearing system. Different solutions have been proposed in the past. In one approach, the sound waves reaching the ear are directly converted into electrical signals which can be accomplished in different ways as described, for example, in U.S. Pat. Nos. 3,882,285, 4,988,333, 5,411,467, and WO 96/21333 and EP 0 831 673. However, with this approach, the natural ability of the outer ear of directionally filtering the received sound is lost and/or the attachment of the required converter components can cause adverse reactions of the affected and surrounding tissue.
In another approach, the natural sound receiving mechanisms of the human outer and middle ear are used for converting the received sound into oscillations of the middle ear components (eardrum and ear ossicle), which are subsequently converted into electrical signals. Different converter principles have been proposed. For example, U.S. Pat. No. 3,870,832 describes implantable converters based on electromagnetic principles. However, the relatively high power consumption of such electromagnetic and electrodynamic converters limits their practical application for cochlear implants and other implantable hearing systems.
This disadvantage is obviated by converters based on piezoelectric principles. EP 0 263 254 describes an implantable converter made of a piezoelectric film, a piezoelectric crystal or a piezoelectric acceleration sensor, whereby one end of the converter is cemented in the bone while the other end is fixedly connected with an oscillating member of the middle ear. The problem with this approach is that inflexible connections to the ear ossicles can cause bone erosion, so that cementing converter components in the middle ear space is approached cautiously for mechanical and toxicological reasons. Moreover, the patent reference does not indicate how the body fluids can be permanently prevented from making contact with the piezoelectric materials. Accordingly, there is a risk of biocompatibility problems, so that the piezoelectric properties can deteriorate due to physical and chemical interactions between the piezoelectric material and the body fluids.
U.S. Pat. No. 3,712,962 describes an implantable converter that uses a piezoelectric cylinder or a piezoelectric beam as a converter component that is anchored in the ear in a manner that is not described in detail. This reference, like the aforementioned patent EP 0 263 254, does not describe in detail how body fluids can be permanently prevented from making contact with the piezoelectric materials.
WO 99/08480 describes an implantable converter based on piezoelectric principles, which is attached solely to an oscillating middle ear component, with the counter support being provided by an inertial mass connected with the converter. However, the attachment of the converter to an oscillating middle ear component, such as the ear drum or the ear ossicles, is either not permanently stable or can erode the bone. This risk is aggravated because the mass of the implantable converter is greater than that of passive middle ear implants.
WO 94/17645 describes an implantable converter based on capacitive or piezoelectric principles, that can be fabricated by micromechanical techniques. This converter is intended to operate a pressure detector in the incudo-stapedial joint. Since the stapes in conjunction with the coupled inner ear forms a resonant system, it may not have sufficient sensitivity across the entire range of useful frequencies. This problem applies also to the implantable converters described in WO 97/18689 and DE 100 30 372 that operate by way of hydro-acoustic signal transmission.
U.S. Pat. No. 3,712,962 describes an implantable converter that uses a piezoelectric converter element that is housed in a hermetically sealed hollow body. The implantable converter is held in position by a support element affixed in the bone channel of the stapes tendon or extended from a screw connection with an ossicle of the middle ear space.
WO 97/11575 describes an implantable hearing aid having a piezo-based microactuator. It includes a disk-shaped transducer which is attached to an end of a tube. The tube is adapted to be screwed into a fenestration formed through the promontory.
U.S. Pat. No. 5,842,967 teaches an implantable contactless stimulation and sensing system utilizing a series of implantable magnets.